Some of the most common congenital heart defects (CHD) in humans are attributable to abnormalities in the cardiac neural crest, a population of ectodermally-derived cells that populates the aortic arch and proximal pulmonary arteries, cardiac outflow tract, and ductus arteriosus, where they form smooth muscle and connective tissue. Ventricular abnormalities, characterized by hypoplastic left or right ventricular chambers, are also commonly seen in newborns. Despite detailed anatomic and physiologic descriptions of these types of CHD, little is known of the molecular mechanisms that control development of the cardiac neural crest or the ventricular chambers. We have discovered two novel basic helix-loop-helix (bHLH) transcription factors, dHAND and eHAND, that are expressed in the cardiac neural crest and developing right and left ventricular chambers, respectively, during embryonic development. The HAND proteins are the earliest chamber-restricted transcription respectively, during embryonic development. The HAND proteins are the earliest chamber-restricted transcription factors identified to date. Inactivation of these genes in mouse or chick embryos result in severe defects in heart development and implicates these factors in chamber specification and neural crest development. The goals of this project are to define the mechanisms that regulate HAND gene expression and to determine the functions of dHAND and eHAND in ventricular and cardiac neural crest development. These studies will be closely interrelated with other projects in this SCOR designed to define the target genes that are transcriptionally regulated by the HAND proteins and the genes that are involved in ventricular and cardiac neural crest defects associated with human CHD.